JP2000295113A - Huffman coded data compressor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the quantity of original data such as data required for expressing a digital picture by preventing a defect in the coding (occurrence of not to be optimized coding). SOLUTION: The compressor 50 that compresses Huffman coded data, is provided with a string detector 52 that detects a set of repetitive strings in received Huffman coded data and a record generator 54 that replaces the set of the repetitive strings with a record including a concrete example of the repetitive strings and denoting the number of the strings in this set.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to data compression of digital images and the like, and more particularly to a Huffman coded data compression apparatus for compressing Huffman coded data.

[0002]

2. Description of the Related Art Generally, a digital image is called an "image element".
Alternatively, it is displayed or printed in the form of a rectangular array corresponding to a “print element”. For the purpose of the present application, "image element" and "print element" are both referred to as "pixel" in this specification. Generally, in a computer, a digital image is represented by one or more arrays of binary numbers. For example, a monochrome digital image can be represented in a computer by a single array of binary numbers.
Each binary number in the array defines the gray level value of the associated pixel. The position of the binary number in the array indicates the spatial position of the pixel.

A color digital image can be represented in a computer by three arrays of binary numbers. Each array (or also referred to herein as the "image plane")
Represents an axis of an appropriate color coordinate system according to the well-known three primary colors theory. The color of the pixels in the digital image is defined by the associated binary number from each array (defining one of the three color components from the color coordinate system). There are many types of color coordinate systems that can be used to represent the color of a pixel.
For example, such color coordinate systems include a "red-green-blue" (RGB) coordinate system, a cyan-magenta-yellow (CMY) coordinate system, and luminescence (luminance).
(Y), red yellow (Cr), blue yellow (C
b) There is a color coordinate system. Usually, the RGB coordinate system is used for a monitor display application, and the CMY coordinate system is used for a print application.

[0004] The amount of data used to represent a digital image can be quite large. For example, 1
Consider a color digital image composed of 024 × 1024 pixels. In a computer, assuming that a pixel is represented by three 8-bit numbers of image planes, a digital image will occupy more than one megabyte of storage space.

[0005] The large amount of data required to represent a digital image in a computer has led to an increase in the required storage capacity and the computational resources and time required to transmit the data to other computing devices. Costs may increase. To reduce these costs,
Various digital image compression techniques have been developed.

In general, digital image compression techniques can be divided into two types: lossless and lossy compression. In lossless compression, a digital image restored after compression matches the original image in pixel units. On the other hand, in the irreversible compression, the image quality of the restored digital image is deteriorated with respect to the original digital image because the compression ratio is higher than the compression ratio of the lossless procedure. One of the widely used lossy compression methods is called “transform coding”. For "Transform coding", see "Digital image Processing, P"
rinciples and Applications ”(pp198-211, ISBN 0-47
1-00949-0 (1994)). Portions of this document are cited herein by reference. A widely used transform coding scheme is Joint Photographic Experts G
It is standardized by publicly available documents and the like issued by roup (JPEG).

[0007] Encoding systems with a transform encoder for encoding a digital image also generally include an entropy encoder that further compresses the data generated by the transform encoder. For this entropy coder, see Pennebake
r, WB and Mitchell, JL "JPEG: Still Imag
e Compression Standard "(pp65-79 and pp189-201,
ISBN 0-442-01272-1 (1993)). Portions of this document are cited herein by reference.

One of the widely used entropy coding techniques is called Huffman coding.
Huffman encoders generally utilize a Huffman statistical model to convert the output of a transform encoder into a series of symbols representing an intermediate representation of a digital image. The Huffman encoder then assigns short codewords (codewords) to the most frequently occurring symbols and longer codewords to those symbols that are less likely to occur. The codewords used in Huffman coding are generally derived from one or more tables known as Huffman tables. The data obtained as a result of Huffman coding represents a digital image in compressed form, which is referred to herein as "Huffman coded data".

[0009]

Although Huffman coding is generally considered to be the best fixed length coding procedure available, the optimal code length is due to the Huffman code being an integer number of bits long. May not be converted. This may cause a problem when a specific character (character) is likely to occur in encoding. This issue of Huffman coding is discussed in Nelson, M. and
he Data Compression Book "(pp113-114, ISBN 1-558
51-434-1 (1995)). Portions of this document are cited herein by reference.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is possible to prevent a problem in encoding which may occur due to the situation of data, and to reduce the original data such as data necessary for representing a digital image. An object of the present invention is to provide a Huffman-encoded data compression device capable of reducing the amount of data.

[0011]

SUMMARY OF THE INVENTION In one embodiment of the present invention, there is provided a compression apparatus for converting received Huffman encoded data into a compressed form. The compression device includes a string detector and a record generator. The compression device is configured to receive Huffman encoded data and the length of the codeword used to generate the Huffman encoded data. When the compressor receives this information, the string detector responds by detecting a set of repeated strings in the Huffman coded data and detects each non-repeated string in the Huffman coded data. For each set of repeated strings found, the record generator responds by replacing the set with the first record. This first record contains data that can be used to recreate the set of repeating strings. Further, for each non-repeating string, the record generator responds by replacing the non-repeating string with a second record. This second record contains data that can be used to reproduce the non-repeating string.

In another embodiment, a method is provided for converting Huffman encoded data to a compressed form. The method comprises the steps of receiving Huffman encoded data;
Detecting each set of repeated strings and each non-repeated string in the Huffman encoded data. Each set of repeating strings is replaced with an associated first record. Further, each non-repeating string is replaced with an associated second record. The first record is
The second record has information that can be used later to reproduce the set of repeating strings, and the second record has information that can be used later to reproduce the non-repeating string.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. The invention of the present application is based on “System For
Compression Of Digital Images Comprising Low Deta
co-pending application entitled “il Areas” and “System For Compr.” attorney docket number 10981977-1.
ession Of Digital Images Comprising Background Pix
Includes content related to a co-pending application entitled "els". These are each cited herein by reference.

In general, the present invention provides a data compression technique that can be used to compress Huffman encoded data. As a first example according to the invention, FIG.
The first embodiment is shown in the form of a compression device 50.
As will be apparent to those skilled in the art, the compression device 50 can be incorporated into many computing devices for the purpose of compressing Huffman encoded data. These computing devices include, but are not limited to, personal computers, scanners, printers, and facsimile machines.

The compression device 50 operates generally to convert a received Huffman encoded data stream into a second data stream representing compressed Huffman encoded data. In this specification, the Huffman encoded data before compression is also referred to as “Huffman data” as appropriate. The Huffman-coded data in the compressed form is referred to as “compressed Huffman data”.

As shown in FIG. 1, the compression device 50 has a string detector 52 and a record generator 54. The compression device 50 also has two inputs, the first
, And a second input 53. During operation of the compression device 52, Huffman encoded data (Huffman data) to be subjected to a compression process is supplied to a first input 51. Further, the length of the codeword (the length of the repeating string or the length of the non-repeating string) used to generate the Huffman encoded data is provided to a second input 53. In the present embodiment, the length of the codeword is obtained from the Huffman table used to generate the Huffman encoded data originally.

Upon receiving the Huffman encoded data and the length of the codeword used to generate the Huffman encoded data, the string detector 52 uses the known codeword length to perform Huffman encoding. Identify repeating and non-repeating strings in the data stream. Then, for each sequence of repeated strings detected, the string detector 52 tells the record generator 54 the length of the repeated string, the number of repeats, and one specific example of the repeated string (instantiation).
) Is generated. Further, for each non-repeating string detected, string detector 52 causes record generator 54 to generate an associated record indicating the length of the non-repeating string and the non-repeating string. Once the records have been generated in this manner, they are each transmitted in a suitable order via output 56 in the form of a compressed Huffman data stream. It should be noted that this data stream can be sent to a memory device for storage, or it can be sent to another computing device for decompression. Also, as will be understood by those skilled in the art, the function of the compression device 50 is provided by dedicated hardware (state mach).
ine) or by microprocessors and software routines.

FIG. 2 shows a second embodiment of the present invention in the form of a stretching device 63. The decompression device 63 can be used to convert compressed Huffman data into non-compressed Huffman encoded data (Huffman data). As shown, the decompression device 63 includes a record reader 64, a string generator 66, an input 62, and an output 65.
have. When the compressed Huffman data is provided at the input 62, the record reader 64 responds by reading and interpreting each record in the compressed Huffman data. For each record read, record reader 64 causes string generator 66 to generate one or more strings indicated by the record to generate Huffman encoded data. Then, the expansion device 63 outputs
Huffman coded data can be transmitted via the H.5 data stream.

One important use of the present invention is to further reduce the data representing a digital image before sending it to a printer for printing. Many printers are configured to receive data contained in a control language format from a host computer. Here, the widely used control language is
"Printer Control language: P
CL). When operating in a PCL environment, the host computer configures the data stream to include both print function commands and interspersed print data.

The printer converts the received data stream into a simple list of commands called display commands that define what to print. The printer then processes the display command and renders the described object into a digital image suitable for printing. This type of digital image is commonly referred to as a raster bit map or rasterized data. Generally, only a small portion of the printer's available memory is allocated to store print function commands and scattered data, and most of the print buffer area is dedicated to processing functions and the resulting raster. Assigned to support bit map images.

On the other hand, there is also a printer system which adopts a procedure in which a host computer rasterizes image data and transmits the rasterized data to a printer. This allows the rasterization of the image to be performed using a host computer processor that generally has greater capabilities than the printer processor. After performing the rasterization, the host computer transmits the rasterized data to the printer.

At times, the rasterized data is first compressed prior to being transmitted to a printer to reduce the time and computational resources required to transmit the rasterized data. Thereby, the capacity of the memory of the printer required for receiving the digital image data can be reduced. In this case, the printer has a decompression capability, and decompresses the rasterized data using the decompression capability. And
After decompression, the rasterized data can be sent to a printer image buffer for printing. An example of such a system is "Page Printer Having Improved System For
Receiving And Printing Raster Pixel Image Data Fro
US Patent No. 5,49, entitled "m A Host Computer".
No. 0,237. Also, "Printing System
m Having Control Language Command And Raster Pixel
It is also shown in U.S. Patent No. 5,706,410 entitled "Image Data Processing Capability". Both of these patents are incorporated herein by reference.

Agent case reference number 1098340-1
`` System For Compression Of Digital Images Compr
co-pending application and attorney docket number 10981977-1 entitled "System Ising Low Detail Areas"
For Compression Of Digital Images Comprising Back
A co-pending application entitled "Ground Pixels" shows a technique that can be used to compress digital image data in a computer before sending it to a printer for printing. In some embodiments of the invention, Huffman encoded data is generated before sending the compressed image data to a printer. Thus, it can be seen that the present invention can be used to further compress image data in these situations. In accordance with the principles of the present invention, computer generated Huffman encoded data (Huffman data) can be converted to compressed Huffman data before sending to a printer.

FIG. 3 shows still another embodiment of the present invention for explaining the features of the present invention. FIG. 3 shows a procedure of an operation for compressing Huffman encoded data. In the present embodiment, the JPEG lossy mode compression standard and the above “System For Compression Of Digital Imag”
Assume that Huffman encoded data has been generated according to the principles taught in the co-pending application entitled "es Comprising Background Pixels."

Referring to FIG. 3, after the Huffman coded data is generated, the length of the Huffman codeword is determined from the Huffman table used to generate the Huffman coded data (step 102). ). The Huffman encoded data is then analyzed using the known lengths of these codewords to identify repetitive and non-repetitive strings in the data (step 104). Each set of repeating strings is then replaced with a single record indicating the length of the repeating string, the number of repetitions, and a specific example of the repeating string (step 106). Further, each non-repeating string is replaced with one record indicating the length of the non-repeating string and the non-repeating string (step 108).
These records form compressed Huffman data and can be sent to the printer (step 11).
0).

FIG. 4 shows a procedure of an operation for expanding Huffman coded data as still another embodiment of the present invention. Upon receiving the compressed Huffman data, each record is read (step 202) and one or more strings represented by those records are generated (step 204). As a result, Huffman encoded data is generated. For example, in a printer, image data relating to the decompressed Huffman encoded data is printed out.

Table 1 shows an excerpt of the Huffman coded data, and illustrates the usefulness of the present invention in reducing the amount of data required to represent a digital image using Table 1. This Huffman encoded data is JPEG
It is generated by compressing uniformly colored digital image data according to the lossy mode compression standard.
It can be seen that this is one situation where Huffman coding is not optimal. That is, this situation is due to the fact that the Huffman code has an integer bit length, as described above, and is likely to generate a single character. As shown in Table 1, the Huffman coded data is a short repetition string “28 A0 028A0
0 ". According to the principle of the present invention, compressed Huffman data composed of one record for all the repetition strings shown in Table 1 can be generated from the Huffman encoded data. As will be appreciated by those skilled in the art, this can significantly reduce the amount of data required to represent the original digital image.

[0028]

[Table 1]

It should be understood that the above description is only illustrative of the invention. All alternatives and modifications may be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.

Hereinafter, embodiments of the present invention will be summarized.

1. Apparatus (50) for compressing Huffman encoded data, comprising: (a) means (52) for detecting a set of repeated strings in the data; and (b) combining the set of repeated strings with one of the repeated strings. Means (54) for substituting a record indicating the number of the strings in the set, the Huffman-encoded data compression apparatus (50), comprising:

2. The Huffman-encoded data compression device according to claim 1, wherein the data represents a digital image.

3. 3. The Huffman coded data compression device of claim 2, wherein the set of repeating strings is associated with a uniformly colored region in the digital image.
0).

4. The Huffman-encoded data compression device (5) according to the item (3), wherein the uniformly colored area is white.
0).

5. The Huffman-encoded data compression device (50) according to claim 4, further comprising means for transmitting the record to a printer.

6. A method of converting Huffman encoded data into a plurality of records, comprising: (a) analyzing the data to identify a set of repeated strings (102).
And (b) replacing each set of repeated strings identified by performing step (a) with an associated record indicating the number of strings in the set and including one instance of the string in the set. (106). A method for converting Huffman-encoded data.

7. (C) analyzing the data to identify each of the non-repeating strings (104);
(D) replacing each of the non-repeating strings identified by performing step (c) with a record indicating the length of the non-repeating string and including the non-repeating string. 7. The Huffman-encoded data conversion method according to the above item 6.

8. (E) The Huffman-encoded data conversion method according to the above (6), further comprising a step (110) of transmitting each record generated by performing the step (b) to the printer.

9. (E) The Huffman-encoded data conversion method according to the above (7), further comprising a step (110) of transmitting each record generated by executing the steps (b) and (d) to a printer.

10. 10. The Huffman-encoded data conversion method according to the above 9, wherein the data represents a digital image.

11. Determining the length of each codeword used to generate the data, wherein the analyzing performed after the determining is performed using the length. Huffman coded data conversion method as described in the above.

[0042]

As described above, according to the present invention, it is possible to prevent a problem in encoding that may occur due to the situation of data, and to reduce data of original data such as data necessary to represent a digital image. There is an effect that a Huffman coded data compression device capable of reducing the amount can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a compression device configured to convert Huffman encoded data into a compressed form according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a decompression device configured to decompress compressed Huffman data according to one embodiment of the present invention.

FIG. 3 is a flowchart showing an operation of compressing Huffman encoded data according to the present embodiment.

FIG. 4 is a flowchart showing an operation of expanding Huffman encoded data according to the present embodiment.

[Explanation of symbols]

 Reference Signs List 50 compression device 52 string detector 54 record generator 63 decompression device 64 record reader 66 string generator

Claims (1)

[Claims] 1. An apparatus for compressing Huffman encoded data, comprising: (a) means (52) for detecting a set of repeated strings in the data; and (b) the set of repeated strings in the data. Means (54) for substituting a record indicating the number of the strings in the set, the Huffman-encoded data compression apparatus (50).